1. Field of the Invention
This invention relates to brush making and in particular to a method for manufacturing a brush having bristles securely anchored in a molded body, and to a brush thus manufactured.
2. Description of the Prior Art
It is common practice in the art of brush making to first mold a brush body with a pattern of bristle-receiving holes and to then secure bristle tufts in the holes by means of wire staples which are rammed into the holes along with looped ends of respective tufts. Although this process produces acceptable brushes, brush makers have long sought to develop a simple process whereby the body is molded directly around the ends of the bristles to secure them in place, thereby eliminating the need for staples.
Stapleless brush making processes have been known since at least 1870, when U.S. Pat. No. 105,373 issued. This patent discloses a brush making process in which a molding material is introduced into the cavity of a mold having perforations in one side through which bristle tufts have been inserted such that looped ends of the tufts project into the cavity. This side of the mold is made of parallel strips which can be separated along the perforations after the molding material forming the brush body has hardened, to facilitate removal of the bristles from the mold side without loosening or extracting the bristles from the body. The use of separable strips is an indication of the inability of the molded back to securely hold the bristles. Such a separable side is also incompatible with the high pressure injection molding techniques used today for high speed brush production, because the areas between facing surfaces of the strips would provide leakage paths for the injected molding material.
Processes for injection molding brush bodies directly around the ends of bristles are disclosed in U.S. Pat. Nos. 2,298,156, 2,317,110 and 3,026,146. In each of these processes a perforated side of the mold, through which bristle tuft ends are inserted into the mold cavity, is formed from a unitary plate. This type of plate eliminates the major leakage paths which exist in a separable plate, but does not eliminate leakage through the perforations around the tufts. Further, to improve bristle retention each of these patents discloses the use of a perforated bristle supporting insert which is positioned inside the mold cavity such that its perforations align with the bristle insertion perforations in the side of the mold. The ends of the bristle tufts are then inserted into the cavity through the aligned perforations and the molding material is injected around the insert, which becomes an integral part of the brush body. While such inserts might improve bristle retention, they cannot be used in the manufacture of small thin brushes. The use of inserts also increases the cost of materials needed to produce brushes, complicates the manufacturing process, and increases the probability of producing defective brushes. The inserts must be specially made, means must be provided to position them in the mold cavity and to align their perforations with those in the perforated mold side, and any deformed or misaligned inserts which prevent proper bristle insertion will cause the respective brushes to be rejected.
Injection molding processes utilizing clamping means for preventing leakage of molding material through bristle insertion perforations are disclosed in U.S. Pat. Nos. 2,643,158 and 3,610,692. In each of these patents, clamping means are forced against the sides of the bristle tufts, where they protrude from the mold perforations, to prevent molding material from leaking through the perforations. Such clamping means add complexity to the mold apparatus, and the applied clamping force must be accurately controlled to prevent permanent deformation or severance of the bristles. U.S. Pat. No. 2,643,158 also discloses a process including fusion of the ends of the bristles in each tuft projecting into the mold cavity to secure the bristles together. To ensure that heat applied to fuse the ends of the bristles does not deteriorate any substantial length of the bristles, heating is effected by bringing an iron into contact with only the very ends of the bristles. Although fusion of the ends of the bristles in this manner might secure individual bristles within a tuft, it does little to improve retention of the tufts themselves.